Thermal imaging, or thermography, is a recording process wherein images are generated by the use of image-wise modulated thermal energy. There are two commonly known methods for thermal imaging. The first is generally referred to as thermal dye transfer printing, and the second is referred to as direct thermal printing. Direct thermal printing involves heating a thermosensitive media to produce a desired image on the thermosensitive media. Thermosensitive media typically comprises a base material, such as polyester, coated with a thermosensitive layer generally containing an organic silver salt. When heated, the organic silver salt is reduced to metallic silver, thereby producing a density at any given location that is a function of the amount of thermal energy provided to the media at the given location.
Application of heat to the thermosensitive media is generally accomplished through use of a thermal recording head or printhead. Thermal printheads typically comprise a number of microscopic heating elements, generally resistors, which are usually spaced in a line-wise fashion across the printhead such that each resistor provides one pixel of a line of pixels produced on the thermosensitive media by the thermal printhead. When printing a desired image, electrical pulses representative of densities corresponding to the desired image are provided to and energize the resistors. Each resistor converts the pulses to thermal energy which is transferred to the thermosensitive media to produce the corresponding pixel at the corresponding density. The image is printed one line of pixels at a time by the thermal printhead as the thermosensitive media is incrementally moved past the thermal printhead by a media transport system.
Most types of thermosensitive media also include a protective layer over the thermosensitive layer. In addition to reducing the occurrence of scratches in the thermosensitive layer, the protective layer includes heat-activated lubricants which are activated during the printing process. Additionally, the protective layer, as well as the thermosensitive layer, “softens” when heated during the printing process. The lubricants and softening of the media reduce the friction between the printhead and media during the printing process. This helps the media to move more easily past the printhead and reduces the occurrence of image artifacts during the printing process. Although thermal printheads generally include a protective coating over the resistors, such as silicon carbide (SiC), for example, reducing friction between the media and the printhead reduces wear on the protective coating and can extend the life of the printhead.
During the printing process, energizing resistors corresponding to pixels beyond the edges of the thermosensitive media can cause such resistors to overheat since the media is not present to absorb and dissipate the generated thermal energy. Such overheating can cause gradual changes in the ohmic value of the resisitor, resulting in a gradual deterioration of image quality. Such overheating may eventually result in failure of the resistors, often referred to as pixel failure. Therefore, to prevent such pre-mature pixel failure and to compensate for slight variations in width and positioning of the media as it moves past the thermal printhead, an edge or border around the perimeter of the media is generally left un-printed.
However, since these border areas of the thermosensitive media are not heated, lubricants are not activated and the media does not soften in the border areas. As a result, there is typically greater friction between the printhead and the media in the border areas than in the printed or “imaged” area. This is especially true when the protective layer includes purposely “raised” particles designed to remove deposits from the printhead. This difference in friction can lead to uneven wearing of the protective coating over the resistors of the printhead and, consequently, to uneven heat transfer characteristics across the width of the printhead. Uneven heat transfer, in-turn, often translates to uneven densities in a printed image.
Uneven wear across a printhead can be especially troublesome when the printhead is used to print images on multiple widths of thermosensitive media. While a narrow width media, in general, will cause a difference in wear across the width a printhead, the wear will be greatest on those areas of the printhead corresponding to the un-printed borders. When printing an image on a media having a greater width, those areas of the printhead may correspond to the imaged area of the media and may produce uneven densities relative to adjacent areas of the printhead as a result of the even wear.
It is evident that there is a need for improving thermal imaging systems, particularly those used to print images on multiple widths of thermosensitive imaging media, to reduce problems associated with un-printed border areas.